In recent years the awareness and concern over the use of petroleum based fuels and lubricants and their impact on the environment has opened the quest for environment friendly lubricants from renewable resources like vegetable oils.
Mineral oil based lubricants, generally suffer from many disadvantages such as high toxicity to the environment and poor biodegradability. There is a need for the environment to be protected from the pollution from lubricants which are not environmentally friendly. Also the dependency on the dwindling supply of petroleum feed stock for the production of mineral oil base stocks is also a cause of worry. For these reasons a new class of environmentally acceptable lubricants is available and vegetable oils find a major place among them along with synthetic esters (polyol esters with short chain fatty acids and diesters) which are considerably expensive and can not be used always. The use of vegetable oils as lubricants is known for a long time. In recent years, increasing attention has been paid to natural triglycerides, fatty acids derived from them, and their potential applications. This raw material is cheap and renewable. Moreover, natural fats and products derived from them are generally environmentally friendly. Their lubricity (antiwear, antifriction and load carrying capacity) characteristics are vastly superior to that of mineral oils and these oils can work at much lower viscosities and remove heat faster than hydrocarbon based lubricants. These lubricants are energy efficient and can cause substantial fuel economy in internal combustion engines (Journal of Synthetic Lubrication, Vol. 23, p. 91, 2006). The operating temperature range of vegetable oils and fatty esters depend upon the degree of saturated chain length and type of alcohol moiety. However, glycerol, a component of the triglyceride molecule is readily destructible at high temperatures. This disadvantageous property stems from the presence of hydrogen atoms in β position relative to the hydroxyl group in the glycerol molecule. This structural feature is conductive to the partial fragmentation of the molecule and the formation of unsaturated compounds. The compounds formed undergo polymerization, increasing the liquid's viscosity and resulting in the formation of precipitate particles. This problem can be solved by replacing glycerol with another polyhydric alcohol which does not contain β-hydrogen atoms, like neopentylglycol (NPG), trimethylolpropane (TMP) or pentaerythritol (Industrial Lubrication and Tribology, Vol. 50, p. 6, 1998). Although such alcohols also decompose at high temperatures, their thermal decomposition has a radical character and proceeds slowly. Synthetic esters produced from vegetable oil based fatty acids can not be used at extremely high temperatures, but they are very suitable in less extreme applications such as two-stroke engine oils, chain bar oils, cutting oils, concrete mould release agents and cosmetic ingredients (Bioresource Technology, Vol. 87, p. 35, 2003).
The properties of esters also depend on the structure of the constituent fatty acids and alcohols, i.e., on the length of their aliphatic chain and the number and relative position of unsaturated bonds. Saturated acids are highly resistant to oxidation and high temperature, but their pour point is high due to the linear structure of the acids. Polyunsaturated fatty acids, especially those containing conjugated bonds, are the most susceptible to oxidation and thermal degradation, even though their esters exhibit lower pour points. The best option is to use mono unsaturated fatty acids like oleic acid or other mono unsaturated fatty acids for the manufacture of synthetic oils. Utilisation of non-edible oil fatty acids for the development of lubricant base stocks is recommended due to the shortage of edible oils. One of the most potential non-edible oil is castor oil.
Review article on plant-oil-based lubricants published in Journal of the Science of Food and Agriculture, Vol. 86, p. 1769, 2006 highlights the advantages and disadvantages of plant based lubricants and various chemical modifications like modification of the ester moiety and multiple bonds to improve the undesirable properties of native plant oils.
High performance ester lubricants from natural oils have been reported in Industrial Lubrication and Tribology, Vol. 54, p. 165, 2002. A new class of bio-based esters derived from vegetable oils that exhibit excellent low temperature properties and oxidation stability are discussed. This is possible due to recent advances in the biotechnology of vegetable oils through advanced plant breeding and genetic engineering and the chemical modifications like changing the chain lengths or introducing branching in the acyl/alkyl chains, changing the polyol backbone structure and mixing asymmetry of the backbone/acyl/alkyl chains to convert these natural esters into high performance biolubricants.
Palmolein blends with Palm oil derived polyol esters comprising palm oil by-products having short chain fatty acids (C6 to C12) with hindered polyols like neopentylglycol, trimethylolpropane and pentaerythritol as biodegradable functional fluids are described in patent (EP 1533360). The limitation is that these fluids are only suitable for tropical climates with temperature ranging from 15-40° C.
R Yunus et al., reported synthesis and characterisation of TMP esters for chemical and lubricant characteristics from palm kernel oil methyl esters (Journal of Oil Palm Research Vol. 15, p. 42, 2003). The lubrication properties of the TMP esters were compared with commercial vegetable oil based TMP esters. The low temperature properties of the esters prepared were inferior to palm oil TMP esters although their lubrication properties were comparable.
Gryglewicz et al. (Bioresource Technology, Vol 87, p. 35, 2003) described the preparation of polyol esters based on vegetable and animal fats, wherein rapeseed oil, olive oil and lard fatty acid methyl esters were transesterified with NPG and TMP using calcium methoxide as catalyst. Esters of lard fatty acids showed higher pour points because of their high saturated acid content.
TMP esters of rapeseed oil fatty acids were synthesized and evaluated as biodegradable hydraulic fluids in comparison to commercially available hydraulic fluids (Journal of the American Oil Chemists' Society, Vol. 75., 1998). These products exhibited good cold stability, friction and wear characteristics and resistance against oxidation at elevated temperatures.
Trimethyolpropane esters useful as lubricant base for motor-car engine were prepared by total esterification of trimethyolpropane with a mixture of saturated aliphatic acids, dicarboxylic acids and iso-acids (U.S. Pat. No. 4,061,581, 1977)
European patent (EP 0712834; 1996) describes the preparation of polyol esters derived from polyols and aliphatic mono carboxylic acid mixtures derived from natural vegetable oils like rapeseed, sunflower, peanut and soybean, wherein the acid mixtures comprise at least about 72% by weight of oleic acid for possible applications like functional fluids and greases.
Flame retardant hydraulic oils containing partial esters of polyols and acyclic mono carboxylic acids having a total of 6-21 carbons have been reported (U.S. Pat. No. 6,402,983, 2002). Even though both undecanoic and 10-undecenoic acids are used for making polyol esters, these esters were reported to be partial polyol esters with hydroxyl value of ≧35.0 mg KOH/g. This clearly indicates that the partial polyol esters prepared essentially contains considerable amount of hydroxyl moiety.
In the prior art for producing lubricants the vegetable oil fatty acids used are either higher fatty acids like oleic, stearic, palmitic or saturated short chain fatty acids like palm kernel and coconut oil fatty acids. The properties of esters depend on the structure of the constituent fatty acids and alcohols. i.e. on the length of the aliphatic chain and the number and relative position of unsaturated bonds. There were no reports on the preparation of total polyol esters based on 10-undecenoic acid and undecanoic acids containing less than 1.0 mg KOH/g hydroxylic value useful for lubricant application.